Spark discharge plugs

ABSTRACT

A spark discharge plug comprises an outer annular electrode and an inner electrode, the two electrodes in use being connected to a source of electric supply, an annular air gap intermediate the electrodes and a dielectric material having a high permittivity interposed between one of the electrodes and the air gap.

United States Patent Chafer et al.

[ June 17, 1975 SPARK DISCHARGE PLUGS Inventors: Henry James Chafer,Hemel Hempstead; Stuart Graham, Coventry, both of England Assignee:Joseph Lucas (Industries) Limited,

Birmingham, England Filed: May 30, 1973 Appl. No.: 365,154

Foreign Application Priority Data June 8, 1972 United Kingdom 26692/72U.S.Cl ..313/l30; 313/131 R;3l3/l37; 313/138; 313/143 Int. Cl. HOlt13/22 Field of Search 313/130, 131 R, 137, 138, 313/131 A [56]References Cited UNITED STATES PATENTS 2,025,914 12/1935 Tetlow 313/1372,833,950 5/1958 Hastings 313/137 2,926,275 2/1960 Peras 313/131 R3,488,556 1/1970 Burley .4 313/131 R Primary Examiner-R. V. RolinecAssistant Examiner-Darwin R. Hostetter Attorney, Agent, or Firm-Holman &Stern 5 7 ABSTRACT A spark discharge plug comprises an outer annularelectrode and an inner electrode, the two electrodes in use beingconnected to a source of electric supply, an annular air gapintermediate the electrodes and a dielectric material having a highpermittivity interposed between one of the electrodes and the air gap.

8 Claims, 1 Drawing Figure SPARK DISCHARGE PLUGS This invention relatesto spark discharge plugs of the kind used in internal combustion enginesfor igniting an air/fuel mixture.

A known form of spark discharge plug comprises an outer annularelectrode and an inner electrode, the two electrodes in use beingconnected to a source of electric supply, and air gap surrounding theinner electrode and an annular body of semi-conductive materialinterposed between the air gap and the outer electrode. With such plugswhen the eletrodes are connected to a source of supply such that the airgap breaks down, a spark occurs which transfers to the outer electrodeso that the main discharge takes place between the electrodes. This formof plug has been known for many years but it has the disadvantage thatthe semiconductive material is eroded by the high temperature to whichit is subjected with the result that the plug has to be replaced atfairly frequent intervals.

The object of the invention is to provide a spark discharge plug in asimple and convenient form.

According to the invention a spark discharge plug of the kind specifiedcomprises an annular outer electrode, an inner electrode, an annular airgap intermediate the electrodes and a dielectric material having apermittivity interposed between one of the electrodes and the air gap,said dielectric material having a high resistance to corrosion.

One example of a spark plug in accordance with the invention will now bedescribed with reference to the accompanying drawing which shows asectional side elevation of the plug for use in the combustion chamberof a gas turbine engine.

With reference to the drawing, there is provided a hollow body formedfrom metal and having a hollow cylindrical extension 11 at the end ofwhich is mounted an annular electrode 12, the electrode having a taperedinternal surface. The body 10 defines an inclined step 13 against whichis located in sealing relationship, a complementary surface formed on aflange 14 on a hollow cylindrical insulator 15 having a skirt 16extending within the body 10. The insulator is retained in position by asleeve member 17 and this in turn is retained by means ofa rolled overportion of the body 10. The insulator is formed for example, fromalumina and a further skirt is provided which extends within the sleevemember. This skirt accommodates, in use, a high voltage lead the core ofwhich is connected to a central electrode 18 of rod like form and havinga head 19 which is brazed to the insulator. Moreover, interposed betweenthe sleeve member 17 and the flange is a compression seal which canaccommodate differences in the thermal expansions of the individualmaterials.

The central electrode 18 has a tip 20 which extends flush with theelectrode 12 and is formed from silicided tungsten. Surrounding thecentral electrode is a further insulator 21 this is formed from materialhaving a high permittivity, at least 50, and which is also resistant tocorrosion by the hot gases resulting from the combustion of the air/fuelmixture within the combustion chamber. Thus insulator 21 has an endportion having tapered surface which co-operates with the taperedsurface of the electrode 12, the insulator being actuated to maintainthe tapered surfaces in contact with each other. Furthermore, theinsulator 21 has an enlargement which cooperates as a sliding fit, withthe bore in the body 10. The electrode 18 extends with a small annularclearance within a bore formed in the insulator 21. This is particularlythe case at the end of the plug. Moreover, by virtue of the taperedsurfaces the thickness of the insulator 21 at the end of the plug isless than at any other position.

In operation, the fact that the insulator is formed from material havinga high permittivity means that the potential across the air gap betweenthe electrode tip 20 and the insulator 21 is greater than the potentialacross the insulator. As a result when the electrodes are connected to asource of supply a substantial proportion of the voltage of the supplyappears across the air gap and when the supply voltage is sufficient,ionisation of the air gap takes place and a spark occurs. The voltageacross the air gap in this form plug is therefore determined not in aresistive manner as is the case with the known form of plug but in acapacitive manner. The voltage required to cause breakdown is higherthan in the known form of plug but the insulator is better able towithstand the effect of the hot gases in the combustion chamber of theengine. One material which is particularly suited for the insulator 21is calcium titanate this material having a value of permittivity of theorder of 100. One such material is that supplied by Stealite andPorcelain Products Limited as TEMPRADES NISOO. This permittivity of at lMc/s and 20c.

We claim:

1. A spark discharge plug for igniting and air/fuel mixture in aninternal combustion engine and comprising an annular outer electrode,and inner electrode, an annular air gap intermediate the electrodes andan a nular body of dielectric material having a permittvity of at least50 interposed between one of the electrodes and the air gap, said bodyacting to ensure that when the electrodes are connected to a source ofelectric supply a substantial proportion of the voltage of the electricsupply appears across the air gap.

2. A spark discharge plug as claimed in claim 1 in which the air gap ispositioned adjacent the inner electrode.

3. A spark discharge plug as claimed in claim 2 in which said dielectricmaterial is calcium titanate.

4. A spark discharge plug as claimed in claim 1 comprising a hollowmetallic body having at one end said outer electrode, the internalsurface of said outer electrode tapering inwardly towards said end ofthe body, a hollow insulator within the body and having an outer endsurface complementary to said tapered portion of the body, means urgingsaid surface of said insulator into engagement with the tapered portionof the body and an electrode extending with clearance through saidinsulator, said insulator being formed from said dielectric material andsaid electrode defining said inner electrode.

5. A spark discharge plug as claimed in claim 4 in which said innerelectrode is mounted on a further insulator carried within the body,said further insulator having a skirt portion surrounding said firstmentioned insulator and a flange having a surface in sealing engagementwith the body.

6. A spark discharge plug as claimed in claim 5 including a sleevemember for retaining said further insulator in position within the bodysaid sleeve member being retained in the body by a rolled over portionof the body.

7. A spark discharge plug as claimed in claim 6 in which said meanscomprises a coiled compression spring.

8. A spark discharge device as claimed in claim 7 in which the materialis calcium titanate.

1. A spark discharge plug for igniting and air/fuel mixture in aninternal combustion engine and comprising an annular outer electrode,and inner electrode, an annular air gap intermediate the electrodes andan annular body of dielectric material having a permittvity of at least50 interposed between one of the electrodes and the air gap, said bodyacting to ensure that when the electrodes are connected to a source ofelectric supply a substantial proportion of the voltage of the electricsupply appears across the air gap.
 2. A spark discharge plug as claimedin claim 1 in which the air gap is positioned adjacent the innerelectrode.
 3. A spark discharge plug as claimed in claim 2 in which saiddielectric material is calcium titanate.
 4. A spark discharge plug asclaimed in claim 1 comprising a hollow metallic body having at one endsaid outer electrode, the internal surface of said outer electrodetapering inwardly towards said end of the body, a hollow insulatorwithin the body and having an outer end surface complementary to saidtapered portion of the body, means urging said surface of said insulatorinto engagement with the tapered portion of the body and an electrodeextending with clearance through said insulator, said insulator beingformed from said dielectric material and said electrode defining saidinner electrode.
 5. A spark discharge plug as claimed in claim 4 inwhich said inner electrode is mounted on a further insulator carriedwithin the body, said further insulator having a skirt portionsurrounding said first mentioned insulator and a flange having a surfacein sealing engagement with the body.
 6. A spark discharge plug asclaimed in claim 5 including a sleeve member for retaining said furtherinsulator in position within the body said sleeve member being retainedin the body by a rolled over portion of the body.
 7. A spark dischargeplug as claimed in claim 6 in which said means comprises a coiledcompression spring.
 8. A spark discharge device as claimed in claim 7 inwhich the material is calcium titanate.